fix warning about: DeprecationWarning: invalid escape sequence '\>'

lynse/core_components/fields_cache/expression_parse.py

@@ -200,7 +200,7 @@ def parse_number(value):
 
 
 class ExpressionParser:
-    """
+    r"""
     FieldExpression is a string parser used to parse field expressions. 
 
     It can reassemble the parsing result into a Filter object of LynseDB, which can filter the query result.